Applet: Automatically raise maximum volume when over defined maximum volume

When volume is above the maximum volume set in applet, moving the volume
slider will only lower it to the maximum applet volume.
This change automatically raises maximum volume when over defined
maximum volume (eg. when the volume is set by other volume control app
or when set by the applications itself like VLC), so it is possible to
control volume over the defined maximum value without first raising maximum volume.

Differential Revision: https://phabricator.kde.org/D5695